Building a web application with a UI in a professional context without a frontend build is borderline malpractice. Even a "thin" layer of JS on top requires some degree of dependency management, and I personally have no desire to go back to the days of vanilla CSS, so you need a SASS/SCSS transpiler. Then there's a lot of handy things that frontend builds do, like normalizing SVG icon formats, automatic organization of static assets etc. The fact is the "islands of interactivity" model still requires two builds.